Make liveness of buffers go through `modifies`
Make liveness of buffers go through `modifies`

Now, a (new-gen) buffer of length 0 is never live. Anyway,
this has no consequence so far because nothing much can be done
with buffers of length 0 yet. It might become a problem in the
future if we were to allow equality comparison of buffers of
length 0.

A-Manning/FStar,A-Manning/FStar,FStarLang/FStar,FStarLang/FStar,A-Manning/FStar,FStarLang/FStar